ISTANBUL - The presidents of Turkey, Iran and Russia met in Ankara Monday in the latest trilateral summit to resolve the Syrian civil war.

Launched in 2017 in the capital of Kazakhstan and known as the Astana peace process, summit leaders Recep Tayyip Erdogan, Hassan Rouhani and Vladimir Putin focused on cooperation, despite profound differences on the future of Idlib, the last enclave of Syrian rebels.

"We are in complete agreement in aiming for a lasting political solution for Syria's political unity and territorial integrity," Erdogan said, setting the tone for the one-day gathering.

"We will drain the terrorist swamp east of the Euphrates (in Syria) and carry our efforts in the fight against terrorism to another level," he said.

Ankara designates the YPG as terrorists linked to a Kurdish insurgency inside Turkey. The YPG is a crucial ally in Washington's war against Islamic State.

Last month, Turkish and U.S. generals hammered out an agreement to jointly create a buffer zone in Syria to protect Turkey's frontier from the YPG. Analysts say many details remain unresolved between the two NATO allies.

Erdogan, flanked by Rouhani and Putin, reiterated his threat to unilaterally act against the U.S.-backed militia by the end of the month if a buffer zone has not been created.

Rouhani attacked America's presence in Syria.

"(President Donald) Trump said last year that U.S. troops will pull out of Syria, but the outcome of this promise has been like his other promise -- a lie," Rouhani said. "It is essential that U.S. troops leave the region at once, and the Syrian government establishes sovereignty in the East, north of (the) Euphrates."

Putin backed a Turkish intervention, saying any country has the right to protect its border. But with the condition, any response has to end once the threat is removed, he said, reiterating Syria's sovereignty.

"The important thing is how can Turkey, Iran (and) Russia go ahead and find shared goals," said International relations professor Mesut Casin of Istanbul's Yeditepe University.

"They are aiming to establish and protect the territorial integrity and Syrian sovereignty."

Analysts says simmering Turkish U.S. tensions over the YPG also facilitates another Putin goal.

"Russia's final aim is to weaken NATO-U.S.-Turkey relations," Casin said.

Syrian constitution, future of Idlib

The three leaders also announced that an agreement had been reached on a committee to draft a new Syrian constitution. The process had been deadlocked for months over disagreements on committee membership. Putin refused to give a date for the committee's first meeting.

Differences also remain over the future of Idlib. Erdogan is urgently lobbying for an end to the ongoing assault by Damascus forces against the rebels.

Ankara fears the assault will lead to another significant exodus of refugees into Turkey. Last week, the Turkish Red Crescent said around half a million Syrians in Idlib had fled to Turkey's border.

Putin robustly defended the assault.

"A zone of deescalation should not serve as a terrain for armed provocations," he said at the beginning of the summit. "We must take supplementary measures to destroy the terrorist menace that comes from the zone of Idlib."

Last year, Erdogan and Putin struck a deal that averted a Syrian army attack on Idlib. Part of the agreement included Turkish forces establishing 12 observation posts to enforce a buffer zone between Damascus forces and rebels.

Terrorist groups were excluded from the agreed upon cease-fire. Ankara and Moscow remain at loggerheads over which groups are terrorists. Rouhani also voiced the need for Damascus forces to take back control of Idlib.

Control of Idlib

"The endgame is whether it's the desire of Ankara or not," said former Turkish diplomatic Aydin Selcen, "that these 12 (Turkish military) observation posts are surrounded and protected by Russian military police, and then (the) Syrian army take over (the entirety) of Idlib, with a 5-kilometer strip left along the Turkish border, and with Turkey already strengthening its border wall, with the plan apparently not to let the refugees into Turkey."

Some analysts suggest Erdogan could be seeking to make a deal, giving up support of the rebels in Idlib in exchange for Russian and Iranian backing to create a zone of control against the YPG militia in northeastern Syria, an area where Erdogan claimed Monday that as many as 3 million Syrians living in Turkey can be resettled.